1. Field of the Invention
The present invention relates to a variable focusing lens that includes three lens groups and is provided in, for example, a projection display device, and a projection display device including the variable focusing lens, more particularly, to a projection variable focusing lens and a projection display device that enlarge and project light having image information from a light valve of, for example, a transmissive or reflective liquid crystal display device or a DMD (digital micro-mirror device) display device onto a screen.
2. Description of the Related Art
In recent years, projection display devices using light valves, such as a liquid crystal display device or a DMD display device, have come into widespread use. In particular, a projection display device has been widely used which uses three light valves corresponding to illumination light components of three primary colors, such as R, G, and B, to modulate the illumination light components, and combines the light components modulated by the three light valves using, for example, a prism, and displays an image on a screen using a projection lens.
As the size of the light valve is reduced, the precision thereof is improved, and personal computers are widely spread, the use of the projection display device for presentation is increased. It is preferable that the projection display device have high user convenience and be easy to install. Therefore, there is an increasing demand for a projection display device having a high performance, a high magnifying power, a small size, and a light weight. In addition, there is an increasing demand for a projection lens having a high performance, a high magnifying power, a small size, and a light weight. Further, there is an increasing demand for a projection lens with a low manufacturing cost.
When a color composition prism for composing the modulated light components from a plurality of light valves or a TIR prism for separating illumination light from projection light is provided in an optical system, the reduction side of the projection lens needs to be substantially telecentric in order to prevent the occurrence of color unevenness in the color composition prism and in order to prevent a reduction in separation efficiency in the TIR prism.
As a projection lens capable of meeting the above-mentioned various demands, various types of projection zoom lenses have been known. The zoom lens generally includes two or more variable power lens groups that are moved together when power varies and a focusing lens group. In the zoom lens, a lens driving unit, such as a cam mechanism, is complicated, and it is difficult to reduce the size, weight, and cost of the zoom lens.
JP-A-2005-300619 discloses a projection zoom lens that uses one lens group to perform zooming and uses another lens group to perform focusing.
However, since the projection zoom lens disclosed in JP-A-2005-300619 also has a zoom function, there are restrictions in the movement of the lens groups when power varies. Therefore, there is a large variation in aberration when power varies.
Further, in the projection zoom lens disclosed in JP-A-2005-300619, the number of lenses arranged on the reduction side in the lens group for varying power is small, and it is difficult to obtain a high zoom ratio in an optical system in which the reduction side is substantially telecentric.